The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency and lowered costs. UMTS Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. However the evolution continues, and a study item on advanced E-UTRAN, hereinafter referred to as LTE-advanced, has recently been approved within 3GPP.
One of several objectives of LTE-advanced, is to increase the spectral efficiency and the system/user throughput, especially at the cell edge. The earlier systems, such as UTRAN and E-UTRAN, which uses orthogonal frequency division multiple access (OFDMA), are basically reuse-1 systems, where adjacent base station sites use the same frequencies, thus allowing efficient use of sparse spectrum resource. But reuse-1 also leads to inter-cell interference, which is mitigated by employing advanced receivers at the base station and at the user equipment (UE). In an LTE-advanced system, the inter-cell interference is expected to be restricted by architectural means in addition to traditional means based on advanced receivers. One such arrangement to be employed in LTE-advanced is called coordinated multiple point transmission/reception (CoMP) system. Another term for CoMP systems, commonly used in the literature, is distributed antenna systems (DAS), but the term CoMP system will be used hereinafter. The introduction of CoMP systems will also impact other processes such as cell search and measurements required for performing cell reselection and handover.
A CoMP system is a network of spatially separated antenna nodes or remote antenna units connected to a common source via a transport network, that provides wireless service within a geographic area. The remote antenna units will hereinafter be referred to as sub base stations. FIG. 1 illustrates the basic concepts of a conventional CoMP system. A controlling unit 110 coordinates the function of all the sub base stations 130. This controlling unit 110 can e.g. be placed in a base station or in a separate centralized controller. The sub base stations 130 within a CoMP system can be passive amplifiers but they can also contain full signal processing capability (i.e. a transceiver). The sub base stations 130 are all tightly synchronized. The area covered by each sub base station 130 is hereinafter referred to as a sub cell 120. Similarly the area covered by a CoMP system will be referred to as a CoMP cell 140.
In LTE-advanced, where CoMP is considered to be an important architecture, each sub base station 130 can be realized by reusing the E-UTRAN base station (eNodeB) in order to ensure the backward compatibility with E-UTRAN. This implies that one CoMP cell could comprise of a set of eNodeBs.
The basic idea with CoMP is to split the transmitted power of a traditional base station among several sub base stations separated in space so as to provide coverage over the same area as the traditional base station but with reduced total power. This leads to a reduction of the inter-cell interference. Typically the UE receives signals from more than one sub base station in a CoMP cell. In other words multiple sub cells 120 serve one UE 150 which will improve the reliability thanks to the diversity gain. Since the sub base stations are all synchronized, the UE will receive data transparently and coherently.
Cell search is a procedure by which the UE finds a cell to potentially connect to in a wireless communication system. Cell search, cell identification and cell detection, which are commonly used terms in mobile communication, all have the same meaning. For consistency reasons cell search and cell search delay will hereinafter be used. As part of the cell search procedure, the UE measures e.g. the reference signal or the synchronization signal from different cells, to find the cells with the strongest received signal. The synchronization signal is also used to determine the cell identity and the frame timing of the found cell as well as the parameters needed for receiving the broadcast channel information. The cell search delay is the time needed for the UE to find and identify a cell. An initial cell search is performed after power up of a UE, whereas neighbor cell search is used to identify candidate target cells for a handover. Neighbor cell search is performed both in connected mode when the UE receives downlink data from the network and in idle mode.
In a CoMP system, common channel signals such as synchronization signals and reference signals, are identically transmitted from all sub base stations in a CoMP cell, which means that they are common to all sub cells in the same CoMP cell. The common synchronization signal will for instance allow UEs to determine a CoMP cell identity (at CoMP cell search). Additional synchronization and reference signals specific to each sub cell may also be transmitted in a conventional CoMP system, to enable UEs to uniquely identify each sub cell in a CoMP cell (at sub cell search). The sub cell identities may be reused in neighboring CoMP cells. Thus a sub cell must be identified by a combination of the CoMP cell identity and the sub cell identity.
A UE is typically served by multiple sub cells, belonging to one serving CoMP cell. The coordinated transmission from multiple sites increases the reliability and reduces the inter-cell interference. One of the roles of the controlling unit in a conventional CoMP system, is to coordinate the transmission from the sub cells, in order to get an optimal system performance. To be able to do this, the UE is required to find and identify sub cells appropriate for its transmission and reception in the CoMP cell (i.e. perform a sub cell search) and report the identified sub cells to the controlling unit.
In a network comprising multiple CoMP cells, the UEs will also move between different CoMP cells. Thus handover has to be supported between CoMP cells, and the CoMP cells are regularly searched to identify the list of candidate target CoMP cells. In order to prevent increased inter-cell interference and lowered performance after a handover from a serving to a target CoMP cell, the target CoMP cell should immediately start serving the UEs from multiple sub cells. This can be realized by letting the serving CoMP cell provide the target CoMP cell with a list of the “strongest” sub cells of the target CoMP cell (i.e. the ones providing the strongest signal) at the time of handover. The UE must thus search the sub cells of each candidate target CoMP cells regularly, and must transmit the identities of the strongest candidate target CoMP cells and the identities of their corresponding strongest sub cells to the serving CoMP cell. In this way the target CoMP cell can resume the transmission to the UE from the strongest sub cells at handover without any additional delay.
The conventional cell search procedure of a UE in connected mode doing a handover from a serving to a target CoMP cell is described hereafter with reference to FIG. 2. A UE 230 is served by a serving CoMP cell 210 and a number of sub cells 220 within this serving CoMP cell 210. The UE 230 will continuously measure the signal strength received from neighboring CoMP cells and based on these measurements transmit a list of the strongest neighboring CoMP cells—i.e. the candidate target CoMP cells 240a-b—to the serving CoMP cell 210. For each one of the candidate target CoMP cells 240a-b, the UE 230 will also do a sub cell search and transmit the identities of the found strongest sub cells (i.e. the target sub cells 250a-b) to the serving CoMP cell 210. This means that at handover from the serving CoMP cell 210 to the candidate target CoMP cell 240a that has been identified as the actual target CoMP cell, the serving CoMP cell 210 can transmit the identities of the target sub cells 250a to the target CoMP cell 240a at once, and the transmission can continue with a minimal disturbance.
The reason for doing a sub cell search in all candidate target CoMP cells is thus to optimize the performance immediately after a handover. If no sub cell search is performed in the candidate target CoMP cells, the CoMP cell that is selected as the target CoMP cell at handover will not be able to assign a set of best sub cells for transmission immediately after the handover. Instead the target CoMP cell will select the sub cells based on uplink transmission from the UE, e.g. RACH transmission, during handover. However due to short RACH burst reception, the selection of sub cells may not be reliable, and the target CoMP cell will then have to reallocate the sub cells when it has received a more reliable signal from the UE. The drawback with performing no sub cell searches at all and instead relying on the RACH transmission, is that the interference in a CoMP cell will increase until correct sub cells are assigned to the UE. In addition the user throughput will degrade due to the delay in the selection of the desired set of sub cells.
The drawback with continuous sub cell searches in all the candidate target CoMP cells is that it will require the UE to perform a large amount of parallel cell searches and to maintain a very long list of sub cell identities. This will increase the UE complexity as more hardware for e.g. processing and storage is needed. Furthermore, when the UEs employ DRX and are in idle mode, the continuous cell search will drain the UE battery.
A further disadvantage with continuous sub cell searches is that the signaling load on the uplink and consequently the processing at the serving CoMP cell is important, as all the sub cell identities need to be transmitted to the controlling unit.